linux-kernel - Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

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Date:   Fri, 2 Oct 2020 08:46:35 -0400
From:   Jonathan Marek <jonathan@...ek.ca>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     freedreno@...ts.freedesktop.org, Rob Clark <robdclark@...il.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent
 cache maintenance

On 10/2/20 3:53 AM, Christoph Hellwig wrote:
>> @@ -8,6 +8,7 @@
>>   #include <linux/shmem_fs.h>
>>   #include <linux/dma-buf.h>
>>   #include <linux/pfn_t.h>
>> +#include <linux/dma-noncoherent.h>
> 
> NAK, dma-noncoherent.h is not for driver use.  And will in fact go
> away in 5.10.
> 

Not actually used, so can be removed.

>>   
>>   #include <drm/drm_prime.h>
>>   
>> @@ -808,6 +809,20 @@ int msm_gem_cpu_fini(struct drm_gem_object *obj)
>>   	return 0;
>>   }
>>   
>> +void msm_gem_sync_cache(struct drm_gem_object *obj, uint32_t flags,
>> +		size_t range_start, size_t range_end)
>> +{
>> +	struct msm_gem_object *msm_obj = to_msm_bo(obj);
>> +
>> +	/* TODO: sync only the required range, and don't invalidate on clean */
>> +
>> +	if (flags & MSM_GEM_SYNC_CACHE_CLEAN)
>> +		sync_for_device(msm_obj);
>> +
>> +	if (flags & MSM_GEM_SYNC_CACHE_INVALIDATE)
>> +		sync_for_cpu(msm_obj);
> 
> And make to these ones as well.  They are complete abuses of the DMA
> API, and while we had to live with that for now to not cause regressions
> they absoutely must not be exposed in a userspace ABI like this.
> 

How do you propose that cached non-coherent memory be implemented? It is 
a useful feature for userspace.